232 WILLIAM THOMSON
removed the necessity for assuming that the sun is cooling
rapidly. Its store of heat may be replenished steadily for
thousands of millions of years by the annihilation and con-
version into heat of its material. With reference to
Thomson's partly unsuccessful essay into cosmogony, it is
interesting to note that the modern exponents of the expand-
ing universe are tending to repeat his difference with other
scientists over the age of the earth. The theory seems to
show that if the universe has always been expanding at its
present rate it cannot be more than a few thousand million
years old. This seems to be far too short a period for the
evolution of nebulae into stars and other cosmical evolutions.
But the example of Thomson makes the modern cos-
mogonists more cautious in concluding that their deductions
of a surprisingly small age for the universe are correct.
In the early fifties Thomson's work was not restricted to
the independent foundation of the science of thermo-
dynamics. His great paper On Transient Electric Currents
was published in 1853. The history of this paper provides
a dramatic illustration of the scope and limitations or
Thomson's genius. He determines with superb clarity and
power "the motion of electricity at any instant after an
electrified conductor of given capacity is put in connexion
with the earth by means of a wire or other linear conductor
of given form and given resisting power." He shows that
if the capacity of the earth is negligible compared with the
capacity of the conductor, and the conductor is without
sensible resistance, the problem depends on one variable
which can be calculated from a consideration of the energy
of the system. The equation for the variable is precisely
the same as that "applicable to the circumstances of a pen-
dulum drawn through a small angle from the vertical and
let go in a viscous fluid, which exercises a resistance simply
proportional to the velocity of the body moving through it.
The interpretation of the solution indicates two kinds or
discharge presenting very remarkable distinguishing charac-
teristics: a continued discharge and an oscillatory dis-
charge, one or other of which will take place in any particular
case. In the continued discharge the quantity of electricity